A liquid crystal is a state of the matter intermediate a solid and an isotropic liquid and characterized by long range orientation order and reduced or absent positional order. Liquid crystals can be divided into thermotropic, lyotropic and metallotropic categories. Thermotropic and lyotropic liquid crystals consist of organic molecules, while metallotropic liquid crystals comprise both organic and inorganic molecules. Thermotropic materials exhibit a phase transition into the liquid crystalline phase as a function of temperature change. In lyotropic liquid crystals, phase transitions are a function of both temperature and concentration of the molecules in a solvent. In the case of metallotropic mesophases, the phase transition depends not only on temperature and concentration, but also on the inorganic-organic composition ratio.
A special class of lyotropic liquid crystals of a non-surfactant nature exists which are known as lyotropic chromonic liquid crystals, for convenience referred to hereinafter as “LCLC.” This family of organic molecules is broad and includes pharmaceutical drugs and dyes. Chromonic molecules are considered lyotropic because they form a liquid crystal phase when in solution with an appropriate solvent, generally water. However they are quite different from a typical amphiphilic molecule (such as a surfactants molecule) that is characterized by a polar head and a flexible hydrophobic tail: chromonic liquid crystals have a disk-like or plank-like shape, with a relatively rigid aromatic core and ionic groups at the periphery.
Both amphiphilic and chromonic molecules aggregate in solution. Surfactant-based lyotropic liquid crystals aggregate forming micelles, while chromonic molecules tend to stack face to face, forming polydisperse, rod-like aggregates [J. Lydon, Chromonics, in: Handbook of Liquid Crystals [Wiley-VCH, Weinheim, 1998) v. 2B, p. 981 and Current Opin. Col. Inter. Sci. 3, 458 (1998)]. The aggregation is driven by weak non-covalent interactions such as π-π attraction, and the length of the aggregates depends on concentration and temperature.
Alignment techniques allow controlling the orientation of liquid crystals on substrates treated with aligning materials such as polyimides. Unidirectional rubbing on polyimide-coated substrates is a standard means of aligning thermotropic liquid crystals in the display industry. Chromonics may be aligned using this technique. A specific alignment direction (parallel or perpendicular) of the liquid crystal with respect to the substrate is achieved by choosing polyimides with different properties. Lyotropic surfactant-based liquid crystals usually self-align when placed between two parallel glass plates. The alignment orientation is dictated by the molecules' interaction with the surface and it is generally homeotropic, which means that the director is oriented perpendicular to the bounding plates.